"From the moment the Sangheili joined forces with humans, we have been viewed as a blight on their honor.
The war against the Flood and Covenant is now over. The war against humanity will begin again."

(The black fades to the Arbiter on another ship, watching the cannons. As they fire, the background fades to black again, leaving only the Arbiter and the words shimmering into existence: "Halo: Sangheili")

Basic Plot of the Game:

The Arbiter has returned home to Sangheilos and has regained his position as Kaidon at the Vadam keep, as well as his name-Thel Vadam. Although he is unused to a life without war, he is ready to find a new path when the famed Xytan Wattinree arrives at his keep's gate with a proposition: Destroy the humans, whose alliance has become a foul mark on the Sangheili honor, take Earth and the Forerunner artifacts found there.

While the Elders of Vadam keep find Xytan's words wise, Thel becomes furious and orders the Supreme Fleet Commander out of Vadam. One of Thel's most trusted advisors, Lak, urges him to reconsider. He refuses.

Some time later, Thel is contacted by Rtas Vadum with this news: Xytan has amassed an army and has offer the humans on Earth an ultimatum: give up their homeworld and live under Sangheili dominion, or be destroyed.

Upon hearing this, Thel and Rtas, along with other Sangheili who have grown in their respect for humans, leave Sangheilos for Earth, and upon arriving, the Shipmaster and Arbiter meet with Lord hood and Colonel Ackerson among others and assure the humans that neither of them, nor half of the Sangheili support Xytan. Rtas offers assistance in aiding the humans flee Earth, providing shelter in friendly keeps on Sangheilos.

As Hood fights hard against the idea of leaving Earth, Thel speaks up. Much to the suprise of all, he backs up Hood. He believes the humans should stand and fight, or Xytan will pick them off one by one, succeeding in what Truth failed to do.

Before Rtas can pull him aside to discuss the matter, the Arbiter takes an oath, that he will stand alongside the humans no matter what, and if humanity meets its death, his blood will mix with theirs on Earth.

After they have heard the humans' decisions, Rtas takes a quiet word with Thel. He does not try to break the Arbiter of his oath, but merely points out that Thel will be alone in this war. Rtas cannot begin a civil war among their own species; the civil war of the Covenant has already shaken the Sangheili too strongly. He wishes Thel good fortune and a swift hand in battle.

As Rtas's ship, Shadow of Intent, makes the Slipspace jump to their home, a fleet of Sangheili ships appear over Earth. Xytan has arrived.

The Arbiter hurries back to Hood and Ackerson and the war begins.

As Thel battles against his own on Earth, Rtas Vadum returns to his keep and discovers a distress beacon coming from a nearby system. The system of Arbiter Ripa Moramee's last battle. Recognizing the origin to be human, he leaves on board the Shadow of Intent and takes with him Usze Taham and N'tho Sraom, who fought with the Arbiter and the Master Chief at the Ark.

As they enter Slipspace, Usze realizes that hiding aboard are four Spec-Ops Xytan followers who try to take Shadow of Intent. Although the Rtas, N'tho and Usze manage to eliminate the threat, one of the infiltrators is able to send out a transmission to other followers of Xytan, informing them of the human ship, before he dies.

The race to reach the humans is on.

For the rest of the game, the player is the Arbiter in the first-person-shooter mode as he carries out missions, infiltration, rescue, and demolition among others or Rtas Vadum as he tracks down the beacon to a system where he finds not only the crippled Spirit of Fire but the remains of Forward Unto Dawn and as he fights to protect the humans there.

Gameplay:

For the Arbiter levels, it is almost like Halo: Combat Evolved combined with the Earth levels of Halo 2 and 3. Co-op is back to the Halo: Combat Evolved and Halo 2 type: Arbiter twins, or triplets, or quadruplets

For Rtas levels, the infiltration level is almost like King of the Hill and the rest are like the Library of Halo: Combat Evolved or the Arbiter levels of Halo 2 or the Cortana level of Halo 3. Co-op is the second and third plays Usze and N'tho while the fourth can play a twin of any of the three, until the Master Chief joins them. Then he is the default second player, but it is also changeable for each level. Also the option to play one of the three spartans on the Spirit of Fire is available when you reach them as well.

The Arbiter has both human and Sangheili weapons available, while Rtas, Uzse and N'tho have just Sangheili weapons until they team up with the humans. All four Sangheili can duel wield Energy Swords, Spartans cannot.